Capacitance transducers have been used for accurate, high resolution, length and displacement measurements. Capacitance is a linear, continuous function of capacitor plate area.
Variable area capacitance transducers measure length and displacement by varying the effective area of a capacitor. This is accomplished by interposing a shield between the plates of a variable capacitor. This approach is inherently linear and gives good performance over a wide displacement range (up to 10 inches or more).
Environmental effects (humidity and temperature) and stray and fringe capacitance, and mechanical design details place practical limitations on accuracy. Practical implementations of these devices introduce a reference capacitor and employ capacitance ratios of the variable and reference capacitors rather than absolute capacitance values of the variable capacitor. The displacement of the shield is then proportional to the ratio of capacitances between two capacitors. Air dielectrics are common, so transducer capacitance is quite small, typically less than one picofarad. Accordingly, any stray capacitance adversely affects the measurement. The problem posed by the stray capacitances is met by using a single electrode common to both capacitors and by carrying out measurements by applying a voltage between the two capacitors in such a way that the common electrode is forced to virtual ground when measurements are being taken. The voltage required to reach virtual ground is proportional to the displacement of the shield.
Several transducer configurations are described in the literature: U.S. Pat. Nos. 3,702,957, 3,566,222, and 3,570,003.
The usual configuration involves two concentric cylinders. The inner cylinder acts as a common center electrode. The outer cylinder is split around its circumference to form two tandem cylinders; the first acting as a reference electrode and the second a working electrode. A movable concentric cylinder passes through the annular space between the working and common electrode and changes the effective area of the working electrode in an amount proportional to its displacement.
Environmental effects and stray and fringe capacitance, and mechanical design details continue to place practical limitations on accuracy. The present invention serves to minimize these limitations.